RGBZ pixel cell unit for an RGBZ image sensor

ABSTRACT

An image sensor is described. The image sensor includes a pixel array having a unit cell that includes visible light photodiodes and an infra-red photodiode. The visible light photodiodes and the infra-red photodiode are coupled to a particular column of the pixel array. The unit cell has a first capacitor coupled to the visible light photodiodes to store charge from each of the visible light photodiodes. The unit cell having a readout circuit to provide the first capacitor&#39;s voltage on the particular column. The unit cell having a second capacitor that is coupled to the infra-red photodiode through a transfer gate transistor to receive charge from the infra-red photodiode during a time-of-flight exposure. The unit cell has a back-drain transistor coupled to the infra-red photodiode.

FIELD OF INVENTION

The field of invention pertains generally to imaging technology, and,more specifically, to an RGBZ pixel unit cell for an RGBZ image sensor.

BACKGROUND

FIG. 1 shows the basic elements of an image sensor 100. As observed inFIG. 1, the image sensor includes a pixel array 101 having constituentpixel cells 102. Coupled to the pixel array 101 is a row decoder 103having outputs that couple to rows of pixel cells 102. Sense amplifiers104 are also coupled to the pixel array 101 column outputs. The imagesensor 100 also includes analog-to-digital circuitry 105 coupleddownstream from the sense amplifiers 104. The image sensor 100 alsoincludes timing and control circuitry 106 that is responsible forgenerating clock and control signals that dictate the operation of theimage sensor 100.

SUMMARY

An image sensor is described. The image sensor includes a pixel arrayhaving a unit cell that includes visible light photodiodes and aninfra-red photodiode. The visible light photodiodes and the infra-redphotodiode are coupled to a particular column of the pixel array. Theunit cell has a first capacitor coupled to the visible light photodiodesto store charge from each of the visible light photodiodes. The unitcell having a readout circuit to provide the first capacitor's voltageon the particular column. The unit cell having a second capacitor thatis coupled to the infra-red photodiode through a transfer gatetransistor to receive charge from the infra-red photodiode during atime-of-flight exposure. The unit cell has a back-drain transistorcoupled to the infra-red photodiode.

An apparatus is described having first means for transferring firstcharge from a first photodiode that has received visible light of afirst type into a storage capacitor and reading out a first voltage ofthe storage capacitor at a pixel array column. The apparatus also hassecond means for transferring second charge from a second photodiodethat has received visible light of a second type into the storagecapacitor and reading out a second voltage of the storage capacitor atthe pixel array column. The apparatus also has third means fortransferring third charge from a third photodiode that has receivedvisible light of a third type into the storage capacitor and reading outa third voltage of the storage capacitor at the pixel array column. Theapparatus also has fourth means for transferring fourth charge from afourth photodiode that has received infra red light during atime-of-flight exposure into a second storage capacitor and reading outa fourth voltage of the second storage capacitor at the pixel arraycolumn.

FIGURES

The following description and accompanying drawings are used toillustrate embodiments of the invention. In the drawings:

FIG. 1 shows a depiction of an image sensor (prior art);

FIG. 2 shows a depiction of a visible light pixel cell;

FIG. 3 shows a depiction of a Z pixel cell;

FIG. 4 shows a depiction of a pixel array having an RGBZ pixel;

FIG. 5 shows a first Venn diagram for a first RGBZ pixel unit celldesign;

FIG. 6 shows an embodiment of an RGBZ pixel unit cell that conforms tothe Venn diagram of FIG. 5;

FIGS. 7a and 7b show layout embodiments for the RGBZ pixel unit cell ofFIG. 6;

FIG. 8 shows a second Venn diagram for a second RGBZ pixel unit celldesign;

FIG. 9 shows a first embodiment of an RGBZ pixel unit cell that conformsto the Venn diagram of FIG. 8;

FIGS. 10a and 10b show layout embodiments for the RGBZ pixel unit cellof FIG. 9;

FIG. 11 shows a second embodiment of an RGBZ pixel unit cell thatconforms to the Venn diagram of FIG. 8;

FIGS. 12a and 12b show layout embodiments for the RGBZ pixel unit cellof FIG. 11;

FIG. 13 shows a third embodiment of an RGBZ pixel unit cell that conformto the Venn diagram of FIG. 8;

FIGS. 14a and 14b show layout embodiments for the RGBZ pixel unit cellof FIG. 13;

FIG. 15 shows a method performed by an RGBZ pixel unit cell;

FIGS. 16a through 16g show a method of manufacture for an RGBZ pixelcell;

FIG. 17 shows an embodiment of a camera system;

FIG. 18 shows an embodiment of a computer system.

DETAILED DESCRIPTION

FIG. 2 shows a circuit design 202 for a visible light pixel. As observedin FIG. 2, initially, a capacitor 201 is cleared of its negative chargeby turning on a reset transistor Q1 (which brings the capacitor'svoltage to a supply voltage (V_pixel)). When the capacitor's negativecharge is cleared and a transfer gate transistor Q2 is turned off, anexposure time begins in which a light sensitive photodiode 203 generatesand collects negative charge (electrons) as a function of the intensityof the light that it receives over the exposure time and the length ofthe exposure time.

After the exposure time, the transfer gate transistor Q2 is turned onwhich transfers the negative charge that was collected in the photodiode203 to the capacitor 201. The transfer of the negative charge into thecapacitor 201 affects the voltage of the capacitor (the more negativecharge the capacitor receives 201 the lower its voltage). After thephotodiode's negative charge has been transferred to the capacitor 201,a row select control signal is enabled that turns on a row selecttransistor Q3 which permits a sense amplifier that is coupled downstreamfrom the pixel cell's column output to sense the capacitor voltage. Thereading of the capacitor's voltage is then digitized and used as anindication of the intensity of the light received by the photodiode 203.The process then repeats.

Typically, the row select signal turns on the row select transistor Q3of every pixel cell along a same row in the pixel array. The row selectsignal “scrolls” through the rows of the array to receive the entirearray image. In the case of a “global shutter” mode, the exposure timesare simultaneous across all pixel cells in the array (and the imageshould not have any motion related artifacts). In the case of a “rollingshutter” mode, the exposure times of the pixel cells are staged, e.g.,on a row by row basis (which can permit the existence of motionartifacts).

The existence of the storage capacitor 201 permits the timing of theexposure to be decoupled from the timing of the row select activationand storage capacitor 201 readout. Said another way, after an exposureand the transfer of charge into the storage capacitor 201, the storagecapacitor's voltage can remain for a period of time before being readout. As a consequence, an image sensor architecture that supportsmultiple exposure times per storage capacitor readout are possible. Thatis, as just one example, an image sensor may be constructed that hasthree exposures with three corresponding transfers of charge into thestorage capacitor 201 for every readout of the storage capacitor 201 inaccordance with its row select activation.

FIG. 3 shows a circuit design for a “Z” pixel 302 for image sensors thatcapture depth information using “time-of-flight” techniques. In the caseof time-of-flight image capture, typically, a light source emitsinfra-red (IR) light from a camera system onto an object and measures,for each of multiple pixel cells of a pixel array, the time between theemission of the light and the reception of its reflected image upon thepixel array. The image produced by the time of flight pixels correspondsto a three-dimensional profile of the object as characterized by aunique depth measurement (z) at each of the different (x,y) pixellocations.

As observed in FIG. 3, the Z pixel design 302 includes a storagecapacitor 301, reset transistor Q1, transfer gate transistor Q2,photodiode 303 and row select transistor Q3 that operate similarly asdescribed above with respect to the visible light pixel cell 202. Thetransfer gate transistor Q2 is turned on-and-off during the exposuretime with a clock signal over the course of the exposure. Controllingthe transfer gate transistor Q2 with a clock signal during the exposuretime of the Z pixel 301 is an artifact of the time of flight technique.In a common approach, the same Z pixel 302 is provided with fourdifferent clocks (each separated in phase by 90°) over four differentreset, exposure time and readout sequences. The four different chargecollection readouts are then combined to calculate the time-of-flightdepth value for the pixel.

During the exposure time itself, as mentioned above, the transfer gatetransistor Q2 toggles on-and-off under the control of a clock signal. Assuch, charge is transferred from the photodiode 303 to the storagecapacitor 301 multiples times during the exposure sequence. During thehalf clock cycles when the transfer gate transistor Q2 is off, a“back-drain” transistor Q4 is on to cause charge from the photodiode 304to flow into the V_pixel supply node. The clock that controls theback-drain transistor Q4 is 180° out-of-phase with the clock thatcontrols the transfer gate transistor Q2 so that while one is on, theother is off.

As such, charge flow out of the photo-diode alternates directionback-and-forth between flowing through the transfer gate transistor Q2and flowing through the back-drain transistor Q4 over the course of theexposure time. Note however, that turning the transfer gate transistorQ2 on and off during the exposure time of the Z pixel cell isfunctionally similar to the particular visible pixel cell embodimentsmentioned just above in which there are multiple exposures andcorresponding charge transfers into the storage capacitor 201 per rowselect readout.

FIG. 4 shows an embodiment of a pixel array 401 having a pixel unit cell402 that includes visible light pixels R, G and B and a Z pixel.Although the particular embodiment shows red (R), blue (B) and green (G)pixels as the visible light pixels, other embodiments may use differentcolored pixel schemes (e.g., Cyan, Magenta and Yellow). For simplicitythe remainder of the present application will largely refer to an RGBvisible light pixel scheme.

A challenge in the layout design of the unit cell 402 is to expand thesurface area of the photodiodes (to enhance optical sensitivity) whilepreserving enough semiconductor surface area space to place thetransistors associated with the pixel unit cell circuit design near orwithin the unit cell (e.g., where such proximity is appropriate ornecessary). That is, the more space such transistors consume, the lessspace there will be to collect light and vice-versa, the more spacethere is to collect light, the less space there is to place thetransistors.

As an additional concern, generally, the storage capacitor used for avisible light pixel design should be smaller to reduce noise duringreadout while the storage capacitor used for a Z pixel design should belarger to promote larger infrared detection pixel capacity.

A solution is to have the visible light pixels share the same storagecapacitor, and, introduce a second storage capacitor for the Z pixel.With the visible light pixels sharing the same storage capacitor, thespace that would otherwise be consumed by the eliminated storagecapacitors is preserved. The shared storage pixel can also be made smallconsistent with the desire to reduce readout signal noise for visiblelight pixels. Additionally, the second storage capacitor for the Z pixelcan be made larger than the visible light storage capacitor consistentwith the desire to have larger infrared detection pixel capacity.

FIG. 5 shows a Venn diagram 500 for an RGBZ pixel unit cell design thatis consistent with this approach. As observed in FIG. 5, each of the R,G and B pixels use the same storage capacitor C1. By contrast, the Zpixel has its own storage capacitor C2 that has larger capacitance thancapacitor C1.

FIG. 6 shows an embodiment of a design for an RGBZ pixel unit cell 600that conforms to the Venn diagram 500 of FIG. 5. As observed in FIG. 6,the respective transfer gate transistor Q2_R, Q2_G, Q2_B for each of theR, G and B photodiodes is coupled to the same storage capacitor C1.Apart from the merged RGB pixel circuit 601 exists a separate, differentZ pixel circuit 602. Both the merged RGB pixel circuit 601 and the Zpixel circuit 602 have their own respective readout circuits that arecoupled to the same array column 603. As such, for example, a same senseamplifier may sense the readout voltage from both pixel circuits 601,602.

Readouts of the respective voltages of the C1 and C2 capacitors are madeat different times (i.e., they cannot be simultaneously read out).Additionally, readouts for the visible light photodiodes are made atdifferent times for the different colors because of the sharing of theC1 capacitor. Visible light readouts may be made sequentially, e.g.,first a red readout is made, then a green readout is made, then a bluereadout is made and then the process repeats. Charge collected by the R,G and B photodiodes can be accumulated simultaneously (or serially).

However, a transfer of charge from any particular visible lightphotodiode to capacitor C1 is made in isolation (i.e., if any one of theRGB transfer gate transistors Q2_R, Q2_G, Q2_B is on for the purpose ofmeasuring incident light the other two transfer gate transistors areoff). Thus charge transfers into C1, like readouts from C1, are madesequentially rather than in parallel. Charge collected in a visiblelight photodiode can be “cleared” by enabling both its transfer gatetransistor and the reset transistor Q1_RGB. In this case, multiplevisible light photodiodes can be cleared simultaneously by enabling eachof the Q2_R, Q2_G, Q2_B transistors and the reset transistor Q1_RGB.

According to one embodiment, the following timing scheme is implementedfor the RGB pixel circuit. First, capacitor C1 is cleared of charge byactivating the reset transistor while the row select transistor is off.Charge within one or more of the visible light photodiodes can besimultaneously cleared with C1 by enabling the respective transfer gatetransistor of each photodiode to be cleared. During exposure time of aparticular visible light photodiode its transfer gate transistor is off.Then, charge from the photodiode is transferred into C1 by activatingits transfer gate transistor while the other transfer gate transistorsand the reset and row select transistors are off. The voltage on C1 fromthe first photodiode is subsequently sensed by activating the row selecttransistor Q3_RGB and the reset transistor Q1_RGB being off (all threetransfer gate transistors may also be off). The charge on C1 is thencleared via reset and the process repeats two more times for each of theother photodiodes. The entire process then repeats.

According to one embodiment the following time scheme is implemented forthe Z pixel circuit 602. The voltage on capacitor C2 is cleared byactivating the reset transistor Q1_Z with the row select transistor Q3_Zbeing off. Either of the transfer gate or back-drain transistors Q2_Z,Q4_Z may also be on to clear any charge in the Z photo-diode. Anexposure time then begins in which a first clock signal is applied tothe gate of the transfer gate transistor Q2_Z and a second clock that is180° out of phase with the first clock signal is applied to the gate ofthe back drain transistor Q4_Z. For those portions (e.g., 50%) of thefirst clock cycle that correspond to a logic level that activates thetransfer gate transistor Q2_Z, charge is transferred from the Zphotodiode into C2. For the remaining portions in which the second clocksignal corresponds to a logic level that activates the back draintransistor Q4_Z charge is transferred from the Z photodiode into theVpix voltage supply terminal.

After the exposure time both clock signals are squelched to turn offboth the transfer gate transistor Q2_Z and the back drain transistorQ4_Z and the reset transistor Q1_Z remains off. Subsequently the rowselect transistor Q3_Z is turned on so that the voltage on capacitor C2can be sensed. The process then repeats three more times such that fourdifferent instances of the first clock signal (and corresponding secondclock signal) having phases of 0°, 90°, 180° and 270° have been appliedto the Z pixel circuit. The readouts from capacitor C2 for the fourdifferent phases are processed downstream to determine a depth value forthe Z pixel. The entire process for a next set of 0°, 90°, 180° and 270°clock signals is then repeated for a next Z pixel depth value.

In operation, the RGB pixel cell circuit 601 and the Z pixel cellcircuit 602 may operate in series or in parallel so long as theirrespective readouts are made at different times. Likewise, R, G and Bpixel values may be generated at the same rate as Z pixel values or atdifferent rates. However, in a higher performance embodiment, R, G and Bpixel values are generated more frequently than Z pixel values owing tothe longer Z pixel exposure times (multiple R, G and B pixel readoutsare made during the Z exposure time). If the R, G and B pixels are tohave readouts at a same or slower rate than the Z pixel, the respectivetransfer gate transistors Q2_R, Q2_G, Q2_B for the R, G and B pixels mayalso be used as back-drain transistors to bleed charge off the R, G andB photodiodes that may accumulate, e.g., during the lengthy Z exposure.Here, both the RGB transfer gate transistors Q2_R, Q2_G, Q2_B and theRGB reset transistor Q1_RGB are activated to force the voltage oncapacitor C1 to a Vpix supply voltage level and permit charge to bleedout of a photo-diode into capacitor C1.

FIGS. 7a and 7b shows two different layout embodiments for the R, G, Band Z photodiodes as well as the transfer gate transistors Q2_R,Q2_G_Q2_B, Q2_Z, back drain transistor Q4, C1 and C2 capacitors. Here,as the transfer gate and back drain transistors are directly coupled toa photo-diode they are typically integrated within the samesemiconductor surface area as the photodiodes. The other transistors ofthe RGB pixel circuit and the Z pixel circuit may be located, e.g., atthe periphery of the pixel array or at the periphery or a macro-cell(e.g., a group of RGBZ unit cells) within the surface area of the pixelarray. One or more of the other transistors may also be integratedwithin the RGBZ unit cell at the expense of photodiode surface area.

As observed in FIGS. 7a and 7b , the transfer gate transistors Q2_R,Q2_G, Q2_B for the RGB pixel cell circuit reside at facing corners ofthe R, G and B photodiodes with the C1 capacitor being formed in aregion that exists at, just outside or near the active regions for eachof the three transfer gate transistors Q2_R, Q2_G, Q2_B. The transfergate and back drain transistors Q2_Z, Q4 for the Z photodiode are formedat corners of the Z photodiode other than the corner that faces C1 andthe RGB transfer gate transistors. C2 is formed in a region that existsat, just outside or near the active region of the Z transfer gatetransistor.

As observed in FIGS. 7a and 7b the gate electrode for the Q2_R, Q2_G,Q2_B, Q2_Z and Q4 transistors may be proximately placed near or at edgesof their respective photodiodes. In an embodiment, a first source/drainregion for each of these transistors is spatially integrated with itsrespective photodiode (e.g., by having it's implant reside within thephotodiode). In a same or different embodiment, a second source/drainregion of these transistors is spatially integrated with C1, C2 or Vpixdepending on the specific transistor For example, in the case oftransistor that is coupled to C1 or C2, the transistor's source/drainregion resides within an implant used to form C1 and C2 and/or receivesa contact that is an electrode for C1 or C2. In the case of transistorthat is coupled to Vpix, the transistor's source/drain region receives acontact that is tied to the Vpix supply voltage potential.

FIGS. 7a and 7b are different in terms of which two of the three“available” (i.e., not facing the RGB transfer gate transistors and C1)corners of the Z pixel are consumed by the Z pixel transfer gatetransistor Q2_Z and the back drain transistor Q4_Z. Another embodiment(not shown) corresponds to the embodiment of FIG. 7a with the positionsof the transfer gate transistor Q2_Z and capacitor C2 being swapped withthe position of the back drain transistor Q4. An even further embodiment(also not shown) corresponds to the embodiment of FIG. 7b with thepositions of the transfer gate transistor Q2_Z and capacitor C2 beingswapped with the position of the back drain transistor Q4.

In various embodiments both of the C1 and C2 capacitors may be formed atleast in part as diffusion capacitances. C2 may be made larger than C1by having any of a different implant than C1, having a larger surfacearea than C1 or having more (e.g., extensive, elaborate, larger) metalstructures formed over it than C1 so that it essentially has largerelectrode plate surface area than C1.

FIG. 8 shows a Venn diagram 800 for another RGBZ pixel unit cell inwhich the visible light pixels share the same capacitor C1, and,introduce a second capacitor C2 for the Z pixel. However, unlike theapproach observed in FIG. 5, the Z pixel in the approach of FIG. 8 notonly has its own larger capacitor C2 but also shares in the use ofcapacitor C1 or is at least coupled to capacitor C1 in some way. Indesigns where the Z pixel actually uses C1 as a storage node for chargefrom the Z photodiode, only a single readout circuit may be utilized(i.e., both the RGB and Z pixel circuits use the same readout circuitsince the circuits are coupled at the storage capacitance node).

FIG. 9 shows an embodiment 900 of an RGBZ pixel unit cell that conformsto the Venn diagram of FIG. 8. As observed in FIG. 9, the respectivetransfer gate transistor Q2_R, Q2_G, Q2_B for each of the R, G and Bphotodiodes is coupled to the same storage capacitor C1. Apart from themerged RGB photodiode, transfer gate and C1 capacitance circuit exists aseparate, different Z photodiode, transfer gate and C2 capacitancecircuit. However, the merged RGB circuit and the Z circuit are coupledtogether through transistor Q5 so that charge from the Z photodiode canbe transferred into both capacitors C2 and C1 during exposure of the Zpixel. This has the effect of increasing the capacitance for the Z pixelto C1+C2. In an embodiment, C2 has larger capacitance than C1. Inanother embodiment C1 and C2 have approximately the same capacitance.

The coupling between the RGB and Z circuits through transistor Q5permits the two pixel circuits to share the same readout circuit aswell. Here, transistor Q5 is essentially used to establish a common nodebetween capacitors C1 and C2 during the exposure of the Z pixel andduring the Z pixel readout. As such, in an embodiment, transistor Q5 ison during Z pixel exposure and during Z pixel readout (but, e.g., is offotherwise).

The RGB pixel cell operates largely as described above with respect toFIG. 6. However, note that the use of capacitor C1 by the Z pixelcircuit causes an extra reset of capacitor C1 by the RGB resettransistor. Specifically, after the voltage on capacitor C1 for one ofthe visible light pixels has been read out, a reset of capacitor C1 willtake place to prepare capacitor C1 to receive charge from the Zphotodiode (this reset does not exist in the embodiment of FIG. 6).After the reset, charge is received in capacitor C1 from the Zphotodiode and the voltage on capacitors C1 and C2 is readout. CapacitorC1 is then reset again to prepare it to receive charge from a visiblelight photodiode.

The Z pixel cell also operates largely as described above with respectto FIG. 6. Here, however, as mentioned just above, during the exposureof the Z pixel, transistor Q5 is also activate to permit the transfer ofcharge from the Z photodiode into capacitor C1. Transistor Q5 remainsactivate until the voltage on capacitors C1 and C2 is readout.

Because of the coupling between the RGB pixel cell and the Z pixel cell,the two cells are less able to operate in parallel and in isolationwithin another as compared to the circuit of FIG. 6. That is, whereasthe RGB and Z pixel circuits 601, 602 of FIG. 6 can operate fullysimultaneously and without reference to one another (other than theycannot be readout simultaneously), by contrast, in the approach of FIG.9, charge from the R, G or B photdiodes can not be transferred into C1during Z pixel exposure. As the Z exposure may take an extended periodof time (because exposures are taken at four different clock phases),back-drain transistors Q4_R, Q4_G, Q4_B are respectively coupled to eachof the R, G and B photdiodes to bleed off charge that they mayaccumulate during the Z pixel exposure. As such, in an embodiment,transistors Q4_R, Q4_G, Q4_B are off during R, G, B pixel exposure butare on during Z pixel exposure.

FIGS. 10a and 10b show RGBZ unit cell layout embodiments for the circuitof FIG. 9. As a point of comparison with the embodiments of FIGS. 7a and7b for the circuit of FIG. 6, the embodiments of FIGS. 10a and 10badditionally show the back drain transistors Q4_R, Q4_G, Q4_B for thevisible light pixels. Transistor Q5 is not coupled directly to aphotodiode and therefore can be placed at the periphery of the array,the periphery of a macro-cell or within the unit cell with acommensurate loss of photodiode surface area size.

FIGS. 10a and 10b are different in terms of which two of the three“available” (i.e., not facing the RGB transfer gate transistors and C1)corners of the Z pixel are consumed by the Z pixel transfer gatetransistor Q2_Z and the four back drain transistors Q4_R, Q4_G, Q4_B,Q4_Z. Another embodiment (not shown) corresponds to the embodiment ofFIG. 10a with the positions of the four back drain transistors Q2_Z andcapacitor C2 being swapped with the position of the back draintransistor Q4_R, Q4_G, Q4_B, Q4_Z. An even further embodiment (also notshown) corresponds to the embodiment of FIG. 10b with the positions ofthe back drain transistor Q4_R, Q4_G, Q4_B, Q4_Z and capacitor C2 beingswapped with the position of the back drain transistor Q4.

Note than in the embodiment of FIG. 10a , the pixel unit cell can beviewed as having “inner halves” of two different Vpix physical nodeslocated on opposite sides of the cell. That is, each Vpix physical nodeis shared by two neighboring pixel unit cells. By contrast, in theembodiment of FIG. 10b , the pixel unit cell can be viewed as having“inner quarters” of four different Vpix physical nodes located at fourdifferent corners of the pixel unit cell. That is, each Vpix physicalnode is shared by four different pixel unit cells.

FIG. 11 shows another embodiment that conforms to the Venn diagram ofFIG. 8. As observed in FIG. 11 the Z pixel cell's use of capacitor C1 isachieved through the back drain transistor Q4. Here, however, thecapacitor C1 is not used as a storage node for the Z photodiode butrather is used as a Vpix supply voltage node. As such, capacitor C1 isused to support the back-drain transistor Q4 according to itsnormal/traditional operation during Z exposure. The C1 capacitor can bebrought to the Vpix voltage level by activating the reset transistorQ1_RGB of the RGB pixel cell circuit. Thus, in an embodiment of FIG. 11,the RGB reset transistor Q1_RGB is made active not only to reset C1prior to an R, G or B exposure, but is also made activate during Zexposure to force capacitor C1 to act as a Vpix voltage node to supportthe operation of the Z pixel back drain transistor Q4.

Restrictions as to when the visible RGB photodiodes can transfer chargeto capacitor C1 and when capacitor C1 can be read out are similar tothose described above with respect to FIG. 9. That is, the RGB pixelcell and Z pixel cell are limited as to how simultaneously they canoperate in isolation to one another. Specifically, an R, G or Bphotodiode can not have its charge transferred to capacitor C1 during Zexposure. The use of C1 as a Vpix voltage node during Z exposure,however, eliminates the extra reset sequence of capacitor C1 (describedabove with respect to FIG. 9) to clear the charge from the Z photodiode.That is, with capacitor voltage being set to Vpix during Z exposure,capacitor C1 effectively remains at a reset level during Z exposure andtherefore is ready to accept charge from an R, G or B pixel immediatelyafter Z pixel exposure and charge transfer into C2.

The embodiment of FIG. 11 has separate readouts for the pair ofcapacitors C1 and C2 since the former is used to accept charge from RGBphotodiodes and the later is used to store charge from the Z photodiode.Unlike the approach of FIG. 9, each of the RGB photodiodes of theapproach of FIG. 11 do not require back-drain transistors because anyoverflow charge accumulated by an RGB photodiode during Z exposure canbe bled off into capacitor C1 through its respective transfer gatetransistor. As such, the RGB transfer gate transistors Q2_R, Q2_G, Q2_Bact not only as transfer gate transistors but also as back-draintransistors. In an embodiment, therefore, the RGB transfer gatetransistors Q2_R, Q2_G, Q2_B are active not only during a chargetransfer for their respective photodiode but also during Z exposure.

FIGS. 12a and 12b show different RGBZ pixel cell layout embodiments forthe circuit design of FIG. 11. As the approach of FIG. 11 does notinclude a back-drain transistor that is coupled to a specially createdVpix supply voltage node, unlike the previous embodiments, the layoutapproach of FIGS. 12a and 12b do not include a Vpix voltage node.Additionally, the back drain transistor Q4 for the Z photodiode isdirectly coupled to capacitor C1. Note that the Z transfer gate Q2_Z canoccupy any of the three corners of the Z photodiode not consumed by theback drain transistor Q4.

It is pertinent to recognize that the various activation/deactivationschemes for various ones of the transistors described above with respectto the circuit design embodiments of FIGS. 6, 9 and 11 may beimplemented by an image sensor timing and control circuit that iscoupled to and provides the control signals to each of these transistorsto establish their on/off state.

Note also that the RGBZ unit cell should appropriately filter light intoits respective pixels. That is, in an embodiment, the R photodiodeshould receive red light by forming a filter structure above it thatsubstantially passes only red light, the G photodiode should receivegreen light by forming a filter structure above it that substantiallypasses only green light, the B photodiode should receive blue light byforming a filter structure above it that substantially passes only bluelight and the Z photodiode should receive infra-red light by forming afilter structure above it that substantially passes only infra-redlight.

FIG. 13 shows two embodiments 1301, 1302 of an RGBZ pixel unit celldesign approach that conforms to the Venn diagram of FIG. 8. As observedin FIG. 13, both embodiments 1301, 1302 include a second transfer gatetransistor Q2_Z_2 to increase the amount of charge collected during Zexposure. That is, recall that in a typical implementation the Zphoto-diode is coupled to a transfer gate transistor and a back-draintransistor. During Z exposure a first clock signal is applied to thetransfer gate transistor and a second clock signal is applied to theback-drain transistor where the first and second clocks are 180° out ofphase. As such, when the first clock turns on the transfer gatetransistor charge is transferred into a storage capacitor that is laterreadout to detect received optical intensity. By contrast, when thesecond clock turns on the back-drain transistor charge is transferredinto a supply node.

The flow of charge into the supply node can be viewed as a loss ofsignal that decreases the sensitivity of the Z pixel. By contrast, theembodiments 1301, 1302 of FIG. 13 apply the first and second clocksignals to first and second transfer gate transistors Q2_Z_1, Q2_Z_2that are coupled to capacitors C2 and C1 respectively. As such, during Zexposure, the charge that traditionally would flow to a supply nodeduring back-draining would instead flow into capacitor C1 when thesecond clock signal turns Q2_Z_2 on. As such, more charge is collectedduring Z exposure resulting in a more sensitive Z pixel.

During readout, the voltage on C1 is sensed and then the voltage on C2is sensed (or first C2 is sensed and then C1 is sensed). Downstream(e.g., with analog, digital or mixed signal circuitry downstream fromthe sense amplifiers) the two voltage readings are combined to produce asingle reading for the pair of clock signals during the Z exposure. Theprocess then repeats, e.g., one more time with clock pairs that are 90°out of phase with the first and second clock signal pair that were justapplied. In an embodiment capacitance C1 is made approximately equal tocapacitance C2 so that same voltage readings from the pair of capacitorsC1, C2 correspond to same amounts of received charge.

With respect to readout, the first embodiment 1301 differs from thesecond embodiment 1302 in that first embodiment 1301 has one readoutcircuit while the second embodiment 1302 has separate readout circuitsfor each of capacitor C1 and capacitor C2. The first embodiment 1301includes transistor Q5 to couple C2 to the readout circuit when thevoltage on capacitance C2 is being sensed. The first embodiment 1301also includes a single reset transistor Q1 to reset both C1 and C2. C1and C2 can be reset (e.g., after both have been readout after a Zexposure) simultaneously by keeping Q5 active during the reset.

In an embodiment, the first embodiment 1301 keeps transistor Q5 offduring Z exposure. During Z exposure the pair of clock signals areapplied to Q2_Z_1 and Q2_Z_2 to alternatively transfer charge into C2and C1 respectively. Additionally, the RGB transfer gate transistors areoff and the RGB back-drain transistors Q4_R, Q4_G, Q4_B are turned on toprevent blooming from the RGB pixels during Z exposure.

When Z exposure is complete Q5 remains off while the voltage on C1 issensed by activating the row select signal (RS) of the readout circuit.After the voltage on C1 is readout, and transistor Q5 is turned on tocouple C2 to the readout circuit. The voltage on C2 is then sensed.After the voltages on C1 and C2 have been sensed, Q5 remains on tocouple both C1 and C2 to the reset transistor Q1. The reset transistorQ1 is then activated to clear the charge on both C1 and C2. TransistorsQ2_Z_2 and Q5 are then turned off to decouple the Z pixel from the RGBpixels, e.g., in preparation for RGB exposure.

During RGB exposure both Z transfer gate transistors Q2_Z_1 and Q2_Z_2are off and the Z back drain transistor Q4_Z is on. The RGB transfergate transistors are activated and the back-drain transistorsdeactivated consistent with embodiments discussed above. For example, inone embodiment, one RGB pixel is exposed while the other two RGB pixelsare not exposed. That is, the back-drain transistor is off and thetransfer gate transistor is on for the RGB pixel being exposed while theback-drain transistor is on and the transfer gate transistor is off forthe other two pixels that are not being exposed. Charge from the exposedphotodiode is transferred into C1 and the voltage on C1 is readout. Thevoltage on C1 is then cleared and, e.g., either another RGB exposure cantake place (for one of the pixels that was not exposed) or a Z exposurecan take place.

In the case of the second embodiment 1302 transistor Q5 is eliminatedbecause of the separate readout and reset circuits. During Z exposuregate transistor Q2_Z_1 is controlled by the first clock signal andcharge flows from the Z photodiode into C2. Also, transfer gatetransistor Q2_Z_2 is controlled by the second clock signal and chargeflows from the Z photodiode into C1. After Z exposure one of the readoutcircuits is activated to sense the voltage on one of the capacitors (theother readout circuit is deactivated). After the first readout, theother readout circuit is activated (the first is deactivated) to sensethe voltage on the second capacitor (the first capacitor may be reset atthis time). After both capacitors have been readout the voltages on thelater or both capacitors are reset. Otherwise, operation is as discussedabove for the first embodiment.

FIGS. 14a and 14b show layout embodiments for the embodiments 1301, 1302of FIG. 13 discussed above. As observed in FIG. 14, R, G and B transfertransistor gates are coupled between the R, G and B photodiodes and C1as in previous layout embodiments. However, unlike previous embodiments,a second Z pixel transfer gate transistor gate Q2_Z_2 is coupled betweenthe Z photodiode and C1. This particular layout approach is achievableat least with the second embodiment. The other observed layout featureshave been discussed in preceding layout embodiments.

FIG. 15 shows a methodology performed by the RGBZ pixel unit cellembodiments discussed above. The method includes transferring chargefrom a first photodiode that has received visible light of a first typeinto a storage capacitor and reading out a first voltage of the storagecapacitor at a pixel array column 1501. The method also includestransferring charge from a second photodiode that has received visiblelight of a second type into the storage capacitor and reading out asecond voltage of the storage capacitor at the pixel array column 1502.The method also includes transferring charge from a third photodiodethat has received visible light of a third type into the storagecapacitor and reading out a third voltage of the storage capacitor atthe pixel array column 1503. The method also includes transferringcharge from a fourth photodiode that has received infra red light into asecond storage capacitor and reading out a fourth voltage of the secondstorage capacitor at the pixel array column 1504.

FIGS. 16a through 16e show a method for manufacturing an image sensorhaving any of the RGB unit cell design strategies discussed above withrespect to FIGS. 4 through 15 and that further includes an RGBZ filterstructure that is consistent with these designs. FIG. 16a shows a crosssection slice of an image sensor along an axis of the pixel array havinga pair of visible light pixels (R and G as observed in FIG. 16a ). Here,the semiconductor substrate 1601 shows the general location of thephoto-sensitive regions of the R and G photodiodes. Metallization 1602deposited on the substrate form the transistor gates and source/draincontacts and metallization deposited above the substrate form thetransistor and circuit interconnect associated with the pixel unit celldesigns and other image sensor circuitry. The metallization andtransistor structures may be formed into any of the various featuresdiscussed above with respect to FIG. 4 through 14 a,b.

After the interconnect metallization formation, as observed in FIG. 16b, a mordent or transparent layer 1603 is formed over the surface overthe underlying structure. Then, visible light filters 1604 of a firsttype (e.g., a red colored “R” filter as observed in FIG. 16b ) areformed in the mordent or transparent layer 1603 by dying the layer theappropriate color in the appropriate region. Specifically, as observedin FIG. 16b , the R pixel regions of the mordent/transparent layer aredyed red. The dying may be implemented by heat transferring a mordentdye through a photoresist mask into a mordent layer and then strippingthe mask, or, imbibing a dye into a transparent layer through aphotoresist mask and then stripping the mask. Here, the photoresist andmasking is patterned so as to expose the regions of interest (the Rpixel regions) and block out the other others regions (the GB and Zregions). Specifically, photoresist is deposited or coated on themordent/transparent layer. The photoresist is then exposed with a maskthat features the R pixel regions. The photoresist is then etched toexpose the underlying transparent/mordent layer in the R regions of thepixel array.

As observed in FIG. 16c , visible light filters of a second type 1605(e.g., a green colored “G” filter as observed in FIG. 16c ) are formedover the interconnect metallization by, e.g., dyeing the appropriate (G)pixel regions of the mordent/transparent layer 1403 green through thetechniques discussed above. FIG. 16d shows the cross section slice of animage sensor along another other axis of the pixel array having visiblelight pixels of a third type (B) and a Z pixel after formation of the Bfilters 1606 according to the same techniques used to form the R and Gpixels discussed above (the already formed R and G filters in thebackground are not depicted for ease of drawing).

As observed in FIG. 16e , the IR filters 1607 are formed in the Z pixelregions of the image sensor. The IR filters pass IR light andsubstantially block visible light. The IR filters 1607 may be formed byforming a photoresist layer over the transparent/mordent layer 1603 andthen exposing the photoresist with a mask that features the Z pixelregions of the pixel array. The photoresist layer is then etched toexpose the transparent/mordent layer in the Z pixel regions. Theunderlying transparent/mordent layer may also be etched in the Z pixelregions and a material that substantially passes only IR light may bedeposited into the exposed regions. The resulting structure is shown inFIG. 16e . Alternatively, the underlying mordent or transparent layermay remain in the Z pixel regions and the IR filter may be deposited ontop of the layer with photo-masking techniques like those describedabove.

In embodiments where the IR filter is formed within a same layer as theRGB filters, the four pixel types may be formed in any order.

After the IR filter is formed, as observed in FIG. 16f , an IR-cutfiltering layer 1608 is deposited or coated on the underlying structureand etched above the Z pixel regions (e.g., using photoresist andmasking techniques). As such, an IR-cut filter is essentially placedover the R, G and B pixel locations. The IR-cut filtering layer 1608 ismade of a material that substantially blocks infrared light. In variousembodiments the IR-cut layer 1608 is helpful because traditional RGBfilters may not substantially block infrared light and, in the contextof an RGBZ image sensor for use in a time-of-flight system, without anIR-cut filter the RGB pixels may respond to the infrared light from thetime-of-flight illuminator. As such, the IR-cut filters help isolate thevisible light and time-of-flight imaging systems.

Additionally the IR-cut filters help prevent the RGB pixels fromsaturating during the Z exposure process which may also alleviate theneed for back-draining the RGB pixels during Z exposure, prevent“blooming” (where oversaturated pixels bleed charge into neighboringpixels) or at least make more straightforward any RGB reset that occursafter a Z exposure. Note that to the extent blooming of the RGB filtersmay remain a concern, back-drain transistors may be coupled to any/allof the RGB photodiodes discussed in various embodiments. As such, forexample, the embodiments of FIGS. 6, 7 a, 7 b and 11, 12 a, 12 b mayadditionally include back-drain transistors for the RGB photodiode andtherefore include the back-drain structures observed in FIGS. 10a and 10b.

As observed in FIG. 16g , micro-lenses 1609 are formed over the filters.Here, a transparent layer (e.g., The micro-lenses can be formed by anyof a number of various processes such as: 1) coating and baking one ormore photoresist layers on the underlying structure, patterning thephotoresist layers into, e.g., circles/cylinders representing themicro-lens array and then melting the photoresist circles/cylinders intothe shape of the micro-lenses; 2) performing the process of 1) above ona layer on a transparent layer (e.g., fused silica) and using the meltedphotoresist as a mask for a RIE etch into the transparent layer (whichcompletes the form of fuller micro-lenses into the transparent layer);3) micro-jetting droplets aimed on the underlying structure in the arraypattern and solidifying the droplets.

FIG. 17 shows an integrated traditional camera and time-of-flightimaging system 1700. The system 1700 has a connector 1701 for makingelectrical contact, e.g., with a larger system/mother board, such as thesystem/mother board of a laptop computer, tablet computer or smartphone.Depending on layout and implementation, the connector 1701 may connectto a flex cable that, e.g., makes actual connection to the system/motherboard, or, the connector 1701 may make contact to the system/motherboard directly.

The connector 1701 is affixed to a planar board 1702 that may beimplemented as a multi-layered structure of alternating conductive andinsulating layers where the conductive layers are patterned to formelectronic traces that support the internal electrical connections ofthe system 1700. Through the connector 1701 commands are received fromthe larger host system such as configuration commands that write/readconfiguration information to/from configuration registers within thecamera system 1700.

An RGBZ image sensor 1703 is mounted to the planar board 1702 beneath areceiving lens 1704. The RGBZ image sensor 1703 includes a pixel arrayhaving an RGBZ unit pixel cell. The RGB pixel cells are used to supporttraditional “2D” visible image capture (traditional picture taking)functions. The Z pixel cells are sensitive to IR light and are used tosupport 3D depth profile imaging using time-of-flight techniques. TheRGBZ pixel unit cell may have RGB pixel cells that share a same storagecapacitor and/or any of the other features discussed above with respectto FIGS. 4 through 16. Although a basic embodiment includes RGB pixelsfor the visible image capture, other embodiments may use differentcolored pixel schemes (e.g., Cyan, Magenta and Yellow).

The image sensor 1703 may also include ADC circuitry for digitizing thesignals from the image sensor and timing and control circuitry forgenerating clocking and control signals for the pixel array and the ADCcircuitry.

The planar board 1702 may include signal traces to carry digitalinformation provided by the ADC circuitry to the connector 1701 forprocessing by a higher end component of the host computing system, suchas an image signal processing pipeline (e.g., that is integrated on anapplications processor).

A camera lens module 1704 is integrated above the RGBZ image sensor1703. The camera lens module 1704 contains a system of one or morelenses to focus received light to the image sensor 1703. As the cameralens module's reception of visible light may interfere with thereception of IR light by the image sensor's time-of-flight pixel cells,and, contra-wise, as the camera module's reception of IR light mayinterfere with the reception of visible light by the image sensor's RGBpixel cells, either or both of the image sensor's pixel array and lensmodule 1703 may contain a system of filters arranged to substantiallyblock IR light that is to be received by RGB pixel cells, and,substantially block visible light that is to be received bytime-of-flight pixel cells.

An illuminator 1705 composed of a light source array 1707 beneath anaperture 1706 is also mounted on the planar board 1701. The light sourcearray 1707 may be implemented on a semiconductor chip that is mounted tothe planar board 1701. A light source driver is coupled to the lightsource array to cause it to emit light with a particular intensity andmodulated waveform.

In an embodiment, the integrated system 1700 of FIG. 17 supports threemodes of operation: 1) 2D mode; 3) 3D mode; and, 3) 2D/3D mode. In thecase of 2D mode, the system behaves as a traditional camera. As such,illuminator 1705 is disabled and the image sensor is used to receivevisible images through its RGB pixel cells. In the case of 3D mode, thesystem is capturing time-of-flight depth information of an object in thefield of view of the illuminator 1705. As such, the illuminator 1705 isenabled and emitting IR light (e.g., in an on-off-on-off . . . sequence)onto the object. The IR light is reflected from the object, receivedthrough the camera lens module 1504 and sensed by the image sensor's Zpixels. In the case of 2D/3D mode, both the 2D and 3D modes describedabove are concurrently active.

FIG. 18 shows a depiction of an exemplary computing system 1800 such asa personal computing system (e.g., desktop or laptop) or a mobile orhandheld computing system such as a tablet device or smartphone. Asobserved in FIG. 18, the basic computing system may include a centralprocessing unit 1801 (which may include, e.g., a plurality of generalpurpose processing cores) and a main memory controller 1817 disposed onan applications processor or multi-core processor 1850, system memory1802, a display 1803 (e.g., touchscreen, flat-panel), a local wiredpoint-to-point link (e.g., USB) interface 1804, various network I/Ofunctions 1805 (such as an Ethernet interface and/or cellular modemsubsystem), a wireless local area network (e.g., WiFi) interface 1806, awireless point-to-point link (e.g., Bluetooth) interface 1807 and aGlobal Positioning System interface 1808, various sensors 1809_1 through1809_N, one or more cameras 1810, a battery 1811, a power managementcontrol unit 1812, a speaker and microphone 1813 and an audiocoder/decoder 1814.

An applications processor or multi-core processor 1850 may include oneor more general purpose processing cores 1815 within its CPU 1801, oneor more graphical processing units 1816, a main memory controller 1817,an I/O control function 1818 and one or more image signal processorpipelines 1819. The general purpose processing cores 1815 typicallyexecute the operating system and application software of the computingsystem. The graphics processing units 1816 typically execute graphicsintensive functions to, e.g., generate graphics information that ispresented on the display 1803. The memory control function 1817interfaces with the system memory 1802. The image signal processingpipelines 1819 receive image information from the camera and process theraw image information for downstream uses. The power management controlunit 1812 generally controls the power consumption of the system 1800.

Each of the touchscreen display 1803, the communication interfaces1804-1807, the GPS interface 1808, the sensors 1809, the camera 1810,and the speaker/microphone codec 1813, 1814 all can be viewed as variousforms of I/O (input and/or output) relative to the overall computingsystem including, where appropriate, an integrated peripheral device aswell (e.g., the one or more cameras 1810). Depending on implementation,various ones of these I/O components may be integrated on theapplications processor/multi-core processor 1850 or may be located offthe die or outside the package of the applications processor/multi-coreprocessor 1850.

In an embodiment one or more cameras 1810 includes an RGBZ image sensorhaving an RGBZ unit cell in which the visible light pixel cells share asame storage capacitor and/or in which the RGBZ unit cell includes anyof the other features discussed above with respect to FIGS. 4 through16. Application software, operating system software, device driversoftware and/or firmware executing on a general purpose CPU core (orother functional block having an instruction execution pipeline toexecute program code) of an applications processor or other processormay direct commands to and receive image data from the camera system.

In the case of commands, the commands may include entrance into or exitfrom any of the 2D, 3D or 2D/3D system states discussed above.

Embodiments of the invention may include various processes as set forthabove. The processes may be embodied in machine-executable instructions.The instructions can be used to cause a general-purpose orspecial-purpose processor to perform certain processes. Alternatively,these processes may be performed by specific hardware components thatcontain hardwired logic for performing the processes, or by anycombination of programmed computer components and custom hardwarecomponents.

Elements of the present invention may also be provided as amachine-readable medium for storing the machine-executable instructions.The machine-readable medium may include, but is not limited to, floppydiskettes, optical disks, CD-ROMs, and magneto-optical disks, FLASHmemory, ROMs, RAMs, EPROMs, EEPROMs, magnetic or optical cards,propagation media or other type of media/machine-readable mediumsuitable for storing electronic instructions. For example, the presentinvention may be downloaded as a computer program which may betransferred from a remote computer (e.g., a server) to a requestingcomputer (e.g., a client) by way of data signals embodied in a carrierwave or other propagation medium via a communication link (e.g., a modemor network connection).

In the foregoing specification, the invention has been described withreference to specific exemplary embodiments thereof. It will, however,be evident that various modifications and changes may be made theretowithout departing from the broader spirit and scope of the invention asset forth in the appended claims. The specification and drawings are,accordingly, to be regarded in an illustrative rather than a restrictivesense.

The invention claimed is:
 1. An apparatus, comprising: an image sensor comprising a pixel array having a unit cell that includes visible light photodiodes and an infra-red photodiode, the visible light photodiodes and the infra-red photodiode coupled to a particular column of the pixel array, the unit cell having a first capacitor coupled to the visible light photodiodes to store charge from each of the visible-light photodiodes, the unit cell having a readout circuit to provide the first capacitor's voltage on the particular column, the unit cell having a second capacitor that is coupled to the infra-red photodiode through a transfer gate transistor to receive charge from the infra-red photodiode during a time-of-flight exposure, the unit cell comprising a back-drain transistor coupled to the infra-red photodiode.
 2. The apparatus of claim 1 wherein the second capacitor is coupled to the readout circuit, the readout circuit to also provide the second capacitor's voltage on the particular column.
 3. The apparatus of claim 2 wherein the unit cell comprises a reset transistor that is coupled to both the first and second capacitors to clear respective charge from the first capacitor and the second capacitor.
 4. The apparatus of claim 2 wherein the visible light photodiodes are coupled to respective transfer gate transistors and respective back-drain transistors.
 5. The apparatus of claim 1 wherein the unit cell comprises a reset transistor that is coupled to both the first and second capacitors to clear respective charge from the first capacitor and the second capacitor.
 6. The apparatus of claim 1 wherein the visible light photodiodes are coupled to respective transfer gate transistors and respective back-drain transistors.
 7. The apparatus of claim 1 wherein the second capacitor is larger than the first capacitor.
 8. The apparatus of claim 1 wherein the unit cell further comprises a second readout circuit coupled to the second capacitor, the second readout circuit to provide the second capacitor's voltage on the particular column.
 9. The apparatus of claim 1 wherein the back-drain transistor is coupled to the first capacitor.
 10. The apparatus of claim 1 wherein the first capacitor is coupled to the second capacitor through a transistor.
 11. A method, comprising: transferring first charge from a first photodiode that has received visible light of a first type into a storage capacitor and reading out a first voltage of the storage capacitor at a pixel array column; transferring second charge from a second photodiode that has received visible light of a second type into the storage capacitor and reading out a second voltage of the storage capacitor at the pixel array column; transferring third charge from a third photodiode that has received visible light of a third type into the storage capacitor and reading out a third voltage of the storage capacitor at the pixel array column; and, transferring fourth charge from a fourth photodiode that has received infra red light during a time-of-flight exposure into a second storage capacitor and reading out a fourth voltage of the second storage capacitor at the pixel array column.
 12. The method of claim 11 further comprising transferring said fourth charge during an exposure time of the fourth photo-diode and performing said transferring of any of said first, second and third charges during said exposure time.
 13. The method of claim 11 further comprising transferring said fourth charge during an exposure time of the fourth photo-diode and permitting fifth charge from any of said first, second and third photo-diodes to flow into a supply node during said exposure time.
 14. The method of claim 11 further comprising transferring said fourth charge during an exposure time of the fourth photo-diode and performing said reading out of any of said first, second and third voltages during said exposure time.
 15. The method of claim 11 further comprising reading out said first, second, third and fourth voltages with a same readout circuit.
 16. The method of claim 11 further comprising clearing said first, second and third voltages from said first capacitor and clearing said fourth voltage from said second capacitor with a same reset transistor.
 17. A computing system, comprising: an applications processor having multiple processing cores coupled to a memory controller, said memory controller coupled to a system memory; a camera system coupled to said applications processor, said camera system comprising an image sensor, said image sensor comprising a pixel array having a unit cell that includes visible light photodiodes and an infra-red photodiode, the visible light photodiodes and the infra-red photodiode coupled to a particular column of the pixel array, the unit cell having a first capacitor coupled to the visible light photodiodes to store charge from each of the visible-light photodiodes, the unit cell having a readout circuit to provide the first capacitor's voltage on the particular column, the unit cell having a second capacitor that is coupled to the infra-red photodiode through a transfer gate transistor to receive charge from the infra-red photodiode during a time-of-flight exposure, the unit cell comprising a back-drain transistor coupled to the infra-red photodiode.
 18. The computing system of claim 17 wherein the second capacitor is coupled to the readout circuit, the readout circuit to also provide the second capacitor's voltage on the particular column.
 19. The computing system of claim 18 wherein the unit cell comprises a reset transistor that is coupled to both the first and second capacitors to clear respective charge from the first capacitor and the second capacitor.
 20. The computing system of claim 18 wherein the visible light photodiodes are coupled to respective transfer gate transistors and respective back-drain transistors.
 21. The computing system of claim 17 wherein the unit cell comprises a reset transistor that is coupled to both the first and second capacitors to clear respective charge from the first capacitor and the second capacitor.
 22. The computing system of claim 17 wherein the visible light photodiodes are coupled to respective transfer gate transistors and respective back-drain transistors.
 23. The computing system of claim 17 wherein the back-drain transistor is coupled to the first capacitor.
 24. The computing system of claim 17 wherein the first capacitor is coupled to the second capacitor through a transistor.
 25. The computing system of claim 17 wherein the second capacitor is larger than the first capacitor. 